How to adjust light's strength in LuxRender?
Firstly, I'm not sure that LuxRender is within the scope of this forum. You might have better luck over at http://www.luxrender.net/forum

That being said, I might still be able to help. LuxRender is very much based on the way light works in the real world. Consequently, adjusting the intensity of the lights is not terribly straightforward.

Under emission settings, "Power" is the wattage of your light source: 40 = ~40 watt light bulb. For that reason, size matters. 40 watts spread over a large area won't be as bright.

Under emission settings, "Efficacy" is lumens per watt. Not sure exactly how this changes the light besides making it lighter or darker, but I have to believe it does something else, too.

Under emission settings, "Gain" is a multiplier for the "Power" setting. Setting gain to 2.0 will double the power of a light source. I have to believe that this isn't the same as just setting the power twice as high, but I don't know what it is.

In short, lowering any of these numbers will make your lightsource dimmer.

HOWEVER, LuxRender is VERY much based on the way light works in the real world. Consequently, your camera isn't just a viewport. It also has an exposure setting, tonemapping, white balance, and other things that will affect how bright the lights appear. If you watch animations done in LuxRender (like this one: https://www.youtube.com/watch?v=SVK7W0SXre4), many of them look like the exposure is being adjusted... because it is, automatically. LuxRender automatically chooses a sensible exposure level based on a number of factors, so you might have to tweak that to get the look you want.

Check these out:

http://www.luxrender.net/forum/viewtopic.php?t=7135

http://blenderdiplom.com/en/tutorials/385-tutorial-introduction-to-lighting-in-luxrender.html

Hope that helps!